The present invention relates to a catalyst for producing a high density polyethylene having a broad molecular weight distribution, in order to obtain good processability and good physical and chemical properties. In particular, the good physical properties may be improved tear properties when the polyethylene is made into films and/or improved environmental stress crack resistance. The present invention further relates to a process for producing said catalyst and to the use of such a catalyst.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distridution (MWD) is a fundamental property which determines the properties of the polymer, and thus its applications. It is generally recognised in the art that the molecular weight distribution of a polyethylene resin can principally determine the physical, and in particular the mechanical, properties of the resin and that the provision of different molecular weight polyethylene molecules can significantly affect the Theological properties of the polyethylene as a whole.
The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution (MWD) is more simply defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. For most applications, the molecular dispersion index varies between 10 and 30.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. These high molecular weight molecules, however render the polymer more difficult to process. On the other hand, a broadening in the molecular weight distribution tends to improve the flow of the polymer when it is being processed at high shear rates. Accordingly, in applications requiring a rapid transformation of the material through a die, for example in blowing and extrusion techniques, the broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (high molecular weight polyethylenes have a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a broad molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion while the high molecular weight portion contributes to a good impact resistance for the polyethylene resin. A polyethylene of this type may be processed using less energy with higher processing yields.
As a general rule, a polyethylene having a high density tends to have a high degree of stiffness. In general, however, the environment stress crack resistance (ESCR) of polyethylene has an inverse relationship with stiffness. In other words, as the stiffness of polyethylene is increased, the environment stress crack resistance decreases, and vice versa. This inverse relationship is known in the art as the ESCR-rigidity balance. It is required, for certain applications, to achieve a compromise between the environmental stress crack resistance and the rigidity of the polyethylene.
Polyethylene is well known in the art for use in making films. Typically, polyethylene films are blown or extruded through a die. The blowing and extrusion of the thin film defines for the film a machine direction in the direction of blowing or extrusion through the die and an orthogonal transverse direction. For many applications, the polyethylene film is required to have a high tear strength, and in particular a high isotropy in the tear strength between the machine and transverse directions. As a result of the blowing or extrusion technique, the polyethylene polymer chains can become substantially aligned in the machine direction of the blowing or extrusion process. This can yield a significantly higher tearing strength in the transverse direction of the film as compared to the tearing strength in the machine direction. There is generally a need for polyethylene films having good tear properties for use in the manufacture of films, and in particular a good isotropy in the tear properties between the machine and transverse directions. A variety of catalyst systems are known for the manufacture of polyethylene. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene resin can vary depending on what catalyst system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced. Thus for example the properties of a polyethylene resin produced using a chromium-based catalyst (i.e. a catalyst known in the art as a xe2x80x9cPhillips catalystxe2x80x9d) tend to be different from the properties of a product employed using a Ziegler-Natta catalyst.
For the manufacture of polyethylene films, it is known that HDPE resins made using Ziegler-Natta catalysts have a good balance in their tear properties between the machine and transverse directions. In particular, such resins made using Ziegler-Natta catalysts, and having what is known in the art as a bimodal molecular weight distribution, have good isotropic tear properties. Such a bimodal HDPE resin has a bimodal distribution of the molecular weight of the high density polyethylene which is represented in a graph of the molecular weight distribution as determined for example by gel phase chromatography. The graph includes in the curve a xe2x80x9cshoulderxe2x80x9d on the high molecular weight side of the peak of the molecular weight distribution. Such a bimodal high density polyethylene consists of high and low molecular weight fractions in which the mixture of those fractions is adjusted as compared to a monomodal distribution so as to increase the proportion of high molecular weight species in the polymer.
The production of high density polyethylene using just a chromium-based catalyst is thus desirable to enable the particular polyethylene product to be manufactured. The Encyclopedia of Polymer Science and Engineering, Volume 6, pages 431-432 and 466-470 (John Wiley and Sons, Inc., 1986, ISBN 0-471-80050-3) and Ullman""s Encyclopedia of Industrial Chemistry, Fifth Edition, Volume A21, pages 501-502 (VCH Verlagsgesellschaft mbH, 1992, ISBN 3-527-20121-1) each discuss Phillips and Ziegler-Natta catalysts and the production of HDPE.
It is known in the art that in order to obtain the advantages of a broad molecular weight distribution, it is necessary to polymerise an intimate mixture of polyethylene molecules prepared in a common manufacturing process, It is known in the art that it is not possible to prepare a polyethylene having a broad molecular weight distribution and the required properties simply by blending polyethylenes having different molecular weights.
It has thus been proposed to carry out the polymerisation by a two step process, using two reactors connected in series (GB-A-1233599; EP-A-057352; U.S. Pat. Nos. 4,414,369 and 4,338,424). In a first step and in the first reactor a fraction of the high density polyethylene is produced under specified conditions and in the following second step in the second reactor a second fraction of the high density polyethylene is produced using a different set of polymerisation conditions. In the two-step process, the process conditions and the catalyst can be optimised in order to provide a high efficiency and yield for each step in the overall process. The currently commercially employed two-step processes suffer from the disadvantage that because two separate serial processes are employed, the overall process has a low throughput.
It has further been proposed to produce polyethylene with a broad molecular weight distribution with a two-catalyst mixture of one supported chromium catalyst and one Ziegler-Natta type catalyst (EP-A-0480376). This process suffers from the disadvantage that the Ziegler-Natta catalyst requires a co-catalyst to give an active catalytic system but the co-catalyst can influence the supported chromium catalyst and in particular can detrimentally affect its activity.
It has also been proposed, for example in EP-A-661299, EP-A-647661 or WO 95/33777 to use chromium-based catalysts for the production of polyolefins.
There is a need in the art for a process for producing polyethylene resins suitable for blow molding having good environmental stress crack resistance (ESCR) and suitable for the manufacture of films having good tear properties, which do not use a Ziegler-Natta catalyst, and in particular which use a chromium-based catalyst.
It is known in the art to provide titanium in a chromium-based catalyst. Titanium can be incorporated either into the support for the chromium catalyst or into the catalytic composition deposited on the support.
Titanium can be incorporated into the support by coprecipitation or terprecipitation as is the case for cogel and tergel type catalysts developed by Phillips Petroleum described for example in EP-A-352715. Cogel and tergel catalysts respectively have binary and ternary supports.
Alternatively, titanium can be incorporated into the support by impregnation of the support as described for example in U.S. Pat. No. 4,402,864 and FR-A-2,134,743 or by chemisorption of a titanium compound into the support as described for example in U.S. Pat. No. 4,016,343.
Titanation of the catalytic composition has been disclosed in earlier patent specifications.
U.S. Pat. No. 4,728,703 discloses that titanium can be incorporated into the catalytic composition by adding to a composite liquid suspension, of a carrier material (i.e. a support) and chromium trioxide, a titanium compound of the formula Ti(OR)4.
U.S. Pat. No. 4,184,979 discloses that titanium can be incorporated into the catalytic composition by adding at elevated temperature a titanium compound such as titanium tetraisopropoxide to a chromium-based catalyst which has been heated in a dry inert gas. The titanated catalyst is then activated at elevated temperature.
The ethylene polymers obtained with all the above mentioned processes do not have satisfactory mechanical properties especially with regard to the environmental stress crack resistance (ESCR).
Therefore there exists a need for a chromium-based catalyst capable of producing polyethylene resins for blow molding, having good processability and good physical and chemical properties.
It is an aim of the present invention to provide a catalyst for the polymerisation of ethylene to produce polyethylene having good processability.
It is another aim of this invention to provide a catalyst for producing ethylene with high environmental stress crack resistance.
It is yet another aim of the present invention to provide a process for producing polyethylene using a chromium-based catalyst, with the resultant polyethylene resin being suitable for the manufacture of polyethylene films and in particular wherein the films have a good balance in their tear properties between the machine and transverse directions.
It is a further aim of the present invention to provide a catalyst for producing polyethylene having the above described desired properties, said catalyst having a high activity.
These and other aims can be achieved with a supported titanated chromium-based catalyst prepared under specific conditions, said catalyst being used for the production of high density polyethylene with improved processability and physical and chemical properties.
The present invention provides a process for preparing a titanated chromium-based catalyst for the production of high density polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, which comprises the steps of;
a) providing a silica-containing support having a specific surface area of at least 400 m2/g;
b) depositing a chromium compound on the support to form a chromium-based catalyst;
c) dehydrating the chromium-based catalyst to remove physically adsorbed water by heating the catalyst at a temperature of at least 300xc2x0 C. in an atmosphere of dry, inert gas;
d) titanating the chromium-based catalyst at a temperature of at least 300xc2x0 C. in an atmosphere of dry, inert gas containing a titanium compound of the general formula selected from RnTi(ORxe2x80x2)m and (RO)n Ti(ORxe2x80x2)m wherein R and Rxe2x80x2 are the same or different and are a hydrocarbyl group containing from 1 to 12 carbon atoms, n is 0 to 3, m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a titanium content of from 1 to 5% by weight, based on the weight of the titanated catalyst and
e) activating the titanated catalyst at a temperature of from 500 to 900xc2x0 C.
The present invention further provides a catalyst for the production of high density polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, the catalyst comprising a silica-containing support having a specific surface area of at least 400 m2/g, a chromium compound deposited on the support, and a titanium compound deposited on the support and comprising from 1 to 5% by weight Ti, based on the weight of the titanated catalyst.
The present invention also provides a process for producing polyethylene, in the presence of a chromium-based catalyst for the production of high density polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, the catalyst comprising a silica-containing support having a specific surface area of at least 400 m2/g, a chromium compound deposited on the support, and a titanium compound deposited on the support and comprising from 1 to 5% by weight Ti, based on the weight of the titanated catalyst.
The present invention further provides the use of the catalyst of the invention in the production of high density polyethylene for providing a high environmental stress crack resistance and a low incidence of melt fracture when melted and subjected to rotational shear at varying speeds.
The present invention further provides the use, for increasing the isotropy of the tear properties of films made from polyethylene resins, of the catalyst system of the invention.
The present invention is predicated on the surprising discovery of the present inventor that, in the production of polyethylene resins, a particular chromium-based catalyst having a minimum specific surface area of a silica-containing support and which has been de-hydrated and the surface titanated prior to or during the process of the activation of the catalyst at elevated temperature,can unexpectedly yield high density polyethylene having a very high environmental stress crack resistance (ESCR) and a low melt fracture index and is able to improve the tear balance of a polyethylene film made from the polyethylene resin.
The silica-containing support material used in the catalyst of this invention can be any catalytic support known in the art. The support is an inorganic, solid, particulate porous material inert to the other components of the catalyst composition and to: any other active components of the reaction system. Thus, suitable supports are inorganic materials, such as silica either alone or in combination with other metallic oxides, e.g., silica-alumina or silica-titania.
The support used in this invention has a large surface area of at least 400 m2/g, preferably from 450 to 600 m2/g and more preferably from 475 to 550 m2/g. The support preferably has a pore volume greater than 1 cm3/g, more preferably from 1 to 3 cm3/g, yet more preferably from 1.3 to 2.5 cm3/g. It is preferred that the support be dried prior to any chromium species being deposited onto it.
Known chromium-containing compounds capable of reacting with the surface hydroxyl groups of the silica-containing supports can be utilised to deposit the chromium thereon. Examples of such compounds include chromium nitrate, chromium trioxide, chromate esters such as chromium acetate, chromium acetylacetonate and t-butyl chromate, silyl chromate esters and phosphorous-containing esters. Preferably, chromium trioxide is used.
A preferred chromium-based catalyst may comprise from 0.5 to 3% by weight of chromium, preferably about 1% by weight of chromium, on a catalyst support, such as a composite silica and titania support.
A particularly preferred chromium-based catalyst for use in the present invention comprises the catalyst xe2x80x9ccatalyst 1xe2x80x9d, having a surface area of 450 m2/g, a pore volume of around 1.5 cc/g and a chromium content of around 1 weight % based on the weight of the chromium-containing catalyst. The support comprises a silica support. Other preferred silica-supported catalysts have a specific surface area of 500 m2/g and respective pore volumes of 2 and 3 cc/g. Another preferred catalyst with a silica support has a specific surface area of 450 m2/g and a pore volume of 1.5 cc/g.
The support is dried by heating or pre-drying of the support with an inert gas prior to use thereof in the catalyst synthesis, in the manner known to those skilled in the art, e.g. at about 200xc2x0 C. for from 8 to 16 hours.
The chromium-based catalyst can be prepared by dry mixing or non-aqueous impregnation but is preferably prepared by impregnation of silica with an aqueous solution of a soluble chromium compound such as CrO3.
The supported chromium-based catalyst is subjected to a pretreatment in order to dehydrate it by driving off physically adsorbed water from the silica or silica-containing support i.e. chemically adsorbed water in the form of hydroxide (xe2x80x94OH) groups bonded to the xe2x80x94Sixe2x80x94Oxe2x80x94 framework of the support need not be removed. The removal of physically adsorbed water avoids the formation of TiO2 as a product from the reaction of water with the titanium compound subsequently introduced during the titanation procedure, as described below. The dehydration step is preferably carried out by heating the catalyst to a temperature of at least 300xc2x0 C. in a fluidised bed and in a dry inert atmosphere of, for example, nitrogen. The dehydration step is preferably carried out for 0.5 to 2 hours.
In a next step, the supported chromium-based catalyst is loaded with a titanium compound. The titanium compound may be of the formula RnTi(ORxe2x80x2)m or (RO)n Ti(ORxe2x80x2)m where R and Rxe2x80x2 are the same or different and can be any hydrocarbyl group containing 1 to 12 carbon atoms, n is 0 to 3, m is 1 to 4 and m+n equals 4. Preferably, the titanium compound is a titanium tetraalkoxide Ti(ORxe2x80x2)4 where Rxe2x80x2 can be an alkyl or a cycloalkyl group each having from 3 to 5 carbon atoms. The titanation is performed by progressively introducing the titanium compound into the stream of dry, inert non-oxidising gas described hereabove in the dehydration step. In the titanation step, the temperature is, as for the dehydration step, maintained at at least 300xc2x0 C. Preferably, the titanium compound is pumped as a liquid into the reaction zone where it vaporises. This titanation step is controlled so that the titanium content of the resultant catalyst is from 1 to 5% by weight, and preferably from 2 to 4% by weight, based on the weight of the titanated chromium-based catalyst. The total amount of titanium compound introduced into the gas stream is calculated in order to obtain the required titanium content in the resultant catalyst and the progressive flow rate of the titanium is adjusted in order to provide a titanation reaction period of 0.5 to 1 hour.
After the introduction of the titanium compound has been terminated at the end of the reaction period, the catalyst is flushed under the gas stream for a period of typically 0.75 hours.
The dehydration and titanation steps are performed in the vapour phase in a fluidised bed.
The titanated catalyst is then subjected to an activation step in dry air at an elevated activation temperature for at least 6 hours. The activation temperature preferably ranges from 500 to 900xc2x0 C., and is most particularly around 650xc2x0 C. Improved ESCR is obtained when the activation temperature is around 650xc2x0 C. The atmosphere is progressively changed from nitrogen to air, and the temperature is progressively increased, from the titanation step to the activation step.
The resultant titanated chromium-based catalyst has a very high activity.
In the preferred polymerisation process of the present invention, the polymerisation or copolymerisation process is carried out in the liquid phase, the liquid comprising ethylene, and where required an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in an inert diluent. The comonomer may be selected from 1-butene, 1-hexene, 4-methyl 1-pentene, 1-heptene and 1-octene. The inert diluent is preferably isobutane. The polymerisation process is typically carried out at a polymerisation temperature of from 85 to 110xc2x0 C. and at a pressure of from 20 to 45 bars. Preferably, the temperature ranges from 95 to 105xc2x0 C. and the pressure from 40 to 42 bars to produce polymer resins of high ESCR. Preferably, the temperature ranges from 90 to 94xc2x0 C. and the pressure is at a minimum of about 24 bars to produce films with improved tear properties.
Typically, in the polymerisation process the ethylene monomer comprises from 0.5 to 8% by weight, typically around 6% by weight, of the total weight of the liquid phase. Typically, in the copolymerisation process the ethylene monomer comprises from 0.5 to 8% by weight and the comonomer comprises from 0 to 4% by weight, each based on the total weight of the liquid phase.
A chemical reducing agent, such as a metal alkyl, may be introduced into the polymerisation reaction. The metal alkyl may comprise triethyl aluminium (TEAI) in an amount of around 0.5 ppm by weight based on the weight of the inert diluent. This can be used when lower activation temperatures for the catalyst have been employed.
Whilst the operating conditions, such as the temperature and pressure of polymerization in the reactor, and the catalyst""s preparation conditions, such as the surface area of the support, obviously have an influence on the properties of the polymer, titanation of the catalyst under the specific conditions described above improves the ESCR, all other factors being substantially equal.
The titanated chromium-based catalyst is introduced into the polymerisation reactor. The alkylene monomer, and comonomer if present, are fed into the polymerisation reactor. In the preferred process of the present invention, the polymerisation or copolymerisation process is carried out in a liquid-full loop reactor; after a residence time in the reactor of 0.5 to 2 hours, and preferably of about one hour, the polyethylene is recovered and transferred to one or more settling legs where the concentration in solids is increased by gravity. The solid content in a loop reactor is typically 30 to 40% by weight; the concentration in a settling leg can be up to 60% by weight. The polymerisation product of high density polyethylene is discharged from the settling legs and separated from the diluent which can then be recycled.
The polyethylene obtained with the catalyst of this invention has a broad molecular weight distribution (MWD) which is represented by the dispersion index D of typically from 12 to 23 and a high density typically from 0.948 to 0.960 g/cm3.
It is surprisingly observed that the polyethylene obtained with the catalyst of this invention has much higher environmental stress crack resistance (ESCR) and a much lower melt index than those obtained using the processes and catalysts of the prior art as summarised above, while keeping similar melt indices and densities. The polyethylene obtained in accordance with the invention also has a very high shear resistance (SR) defined as HLMI/MI2 where HLMI is the high load melt index measured at 190xc2x0 C. and under a load of 21.6 kg and MI2 is the melt index measured at 190xc2x0 C. under a load of 2.16 kg, both with the ASTM D-1238 standard method. The high shear resistance can result in suppression of the melt fracture phenomenon.
The following Examples are given to illustrate the invention without limiting its scope.